1. Field of the Invention
The present invention relates to a transmission power controller for use in a mobile communication terminal equipment.
2. Description of the Prior Art
Control circuits for controlling the maximum average transmission power to be below a certain value that is stipulated under the Radio Related Law generally include the so-called ALC (Automatic Level Control) circuit in conventional mobile communication terminal equipment of an analog FM (Frequency Modulation) system.
FIG. 4 exemplifies an arrangement of this circuit.
VCXO (Voltage Controlled Crystal Oscillator) 21 generates an IF (Intermediate Frequency) signal that is frequency modulated with a modulation signal.
Band pass filter 23 restrains a bandwidth for the output signals from VCXO 21 to eliminate undesired waves and allows the IF signal with a desired frequency to pass through.
IF amplifier 24 amplifies the IF signal output from band pass filter 23. Band pass filter 25 restrains a bandwidth for the output signals from IF amplifier 24 to eliminate undesired waves and allows the IF signal with the desired frequency to pass through.
Mixer 26 mixes the IF signal output from band pass filter 25 with a local signal output from local oscillator 27 to convert the IF signal to a RF (Radio Frequency) signal.
Band pass filter 28 restrains a bandwidth for the output signals from mixer 26 to eliminate undesired waves and allow the RF signal with a desired frequency to pass through. The RF signal output from band pass filter 28 is power-amplified at transmission power amplifier 29 and then radiated from antenna 30 as a transmission radio wave.
Branching filter 31 partly separates the output signal from transmission power amplifier 29. Detector 32 detects the output signal from transmission power amplifier 29 separated by branching filter 31. Smoothing circuit 33 smoothes the output signal from detector 32. Thus obtained is a DC voltage (hereinafter referred to as a detected voltage) corresponding to an average transmission power from the mobile communication terminal equipment including the transmission power controller in FIG. 4.
Threshold setting circuit 34, on the other hand, outputs on the basis of a threshold data signal a threshold voltage corresponding to the maximum average transmission power stipulated under the Radio Related Law.
Voltage comparator 35 compares the detected voltage output from smoothing circuit 33 with the threshold voltage output from threshold setting circuit 34.
A negative feedback control is further performed so as to equalize the average transmission power from the mobile communication terminal equipment with an allowable maximum average transmission power by adjusting at supply voltage controller 36 a supply voltage, bias voltage and the like of transmission power amplifier 29.
The above is the operation of the transmission power controller (ALC circuit) in the mobile communication terminal equipment of the conventional analog FM system.
CDMA (Code Division Multiple Access) is recently proposed as a technology for mobile communication terminal equipment in the next generation.
In order to overcome the Near-Far Problem in the CDMA system, it is necessary to control the transmission power of the mobile communication terminal equipment precisely over a wide range so that radio waves transmitted from mobile communication terminal equipments belonging to the same cell can arrive at a base station with substantially identical signal levels. The range for the control is wider than 80 dB and the resolution for the control is 1 dB.
It is impossible, however, to realize such complicated transmission power control by the ALC circuit as shown in FIG. 4.
In order to overcome this problem, a method of precisely adjusting a control voltage of a variable gain amplifier which operates at IF stage with a wide dynamic range is employed. "RF2609" available from RF Micro Device Inc. is well known as an example of such variable gain amplifier.
FIG. 5 shows an arrangement example of a transmission power controller with such variable gain amplifier. Quadrature modulator 41 is used in CDMA because modulation methods such as QPSK and offset QPSK are employed, instead of VCXO in the mobile communication terminal equipment of the analog FM system.
Quadrature modulator 41 has two base-band inputs: an in-phase component (I-signal); and a quadrature component (Q-signal). Quadrature modulator 41 modulates a local signal output from local oscillator 42 with the I-signal and Q-signal to generate an IF signal.
Band pass filter 43 restrains a bandwidth of the output signal of quadrature modulator 41 so as to eliminate undesired waves and obtain an IF signal with a desired frequency.
Variable gain amplifier 44 amplifies the IF signal output from band pass filter 43. Band pass filter 45 restrains a bandwidth of the output signals from variable gain amplifier 44 to eliminate undesired waves and allows the IF signal with the desired frequency to pass through.
Mixer 46 mixes the IF signal output from band pass filter 45 with a local signal output from local oscillator 47 to convert the IF signal into an RF signal.
Band pass filter 48 restrains a bandwidth of the output signals from mixer 46 to eliminate undesired waves and allow an RF signal with a desired frequency to pass through. The RF signal output from band pass filter 48 is power-amplified at transmission power amplifier 49 and then radiated from antenna 50 as a transmission radio wave.
The circuit of FIG. 5 does not use ALC circuit as shown in FIG. 4. Instead, the circuit of FIG. 5 includes D/A converter 57 to generate a control voltage for variable gain amplifier 44 and controller 56 to set data for D/A converter 57.
Two types of transmission power control, which are an open loop power control and a closed loop power control, are performed in CDMA systems. In the open loop power control, a mobile communication terminal equipment monitors the transmission power from a base station. If the transmission power is relatively higher, the mobile communication terminal equipment decides that the terminal located near to the base station and lowers its own transmission power. If the transmission power from the base station is relatively lower, the mobile communication terminal equipment decides that the terminal located far from the base station and elevates its own transmission power. The open loop power control signal is improved to controller 56 by the CPU of the mobile communication terminal equipment.
In the closed loop power control, the base station monitors the transmission power from the mobile communication terminal equipment. The base station transmits to the mobile communication terminal equipment a control signal for lowering the power if the transmission power is relatively higher and for elevating the power if the transmission power is relatively lower. The mobile communication terminal equipment periodically varies a control value of the transmission power by a certain variable amount such as 1 dB step in accordance with the total power control signal.
Controller 56 comprises an up/down counter 61 as shown in FIG. 6. The aforementioned total power control signal is a signal of 1 bit. When the signal has a value of "1", the signal instructs to elevate the transmission power. When the signal has a value of "0", the signal instructs to lower the transmission power. The total power control signal is fed to U/DB terminal of up/down counter 61 through TPC (Total Power Control) terminal of controller 56.
Up/down counter 61 counts periodically in synchronism with a clock signal fed to the CK terminal and counts up or down when an input to U/DB terminal is "1" or "0". Up/down counter 61 outputs its count value as digital data from the Dout terminal. D/A converter 57 converts the output data to an analog signal and outputs the analog signal as a control signal for variable gain amplifier 44.
The control voltage output from D/A converter 57 controls the gain of variable gain amplifier 44, which in turn controls the transmission power of the mobile communication terminal equipment.
Up/down counter 61 loads the open loop power control signal fed to Din terminal when a load signal is given to LOAD terminal and outputs as digital data from Dout terminal a count value corresponding to the control value of the power that is indicated with the control signal. D/A converter 57 converts the output data of controller 56 to an analog signal and outputs the analog signal as a control signal for variable gain amplifier 44 similarly to the above.
The above-mentioned are the operations of a conventional transmission power controller in a mobile communication terminal equipment of the CDMA system.
The transmission power controller of FIG. 5 causes the following disadvantages.
(1) When a mobile communication terminal equipment is far from a base station, the transmission power of the mobile communication terminal equipment which is monitored at the base station becomes low. Then, the base station transmits a total power control signal instructing to increase the transmission power. Therefore, if the mobile communication terminal equipment is extremely far from the base station, the closed loop power control for increasing the transmission power may be performed excessively. As a result, a transmission power which exceeds the allowable maximum average transmission power may be output sometimes from the mobile communication terminal equipment. In such case, the mobile communication terminal equipment violates the stipulations of the Radio Related Law although it satisfies the requirement from the base station. Operation in such case may also possibly interfere neighboring base stations.
(2) When a mobile communication terminal equipment is far from a base station, the transmission power of the base station which is monitored at the mobile communication terminal equipment becomes low. Then, the CPU of the mobile communication terminal equipment outputs a open loop power control signal instructing to increase the transmission power. Therefore, if the mobile communication terminal equipment is extremely far from the base station, the open loop power control for increasing the transmission power may be performed excessively. As a result, the same disadvantage as that in case of the closed loop power control is caused.
(3) The gain of the variable gain amplifier varies, depending on ambient temperatures. A desired transmission power can not be attained accordingly even if the total power control signal or open loop power control signal is appropriate. As a result, the same disadvantage as that in case of the closed loop power control is caused.